CCalendar::GetOriginalDate

static function GetOriginalDate(
	$parentDateTime,
	$instanceDateTime,
	$timeZone = null,
	$format = null
): string
{
	CTimeZone::Disable();
	$format = $format ?? MainTypeDate::convertFormatToPhp(FORMAT_DATETIME);
	$parentTimestamp = Util::getDateObject($parentDateTime, false, $timeZone)->getTimestamp();
	$baseTimeZone = date_default_timezone_get();
	if ($timeZone)
	{
		date_default_timezone_set($timeZone);
	}
	$parentInfoDate = getdate($parentTimestamp);
	/** @var TypeDateTime $instanceDateTime */
	$instanceDateTime = Util::getDateObject($instanceDateTime, false, $timeZone);
	$eventDate = $instanceDateTime->setTime($parentInfoDate['hours'], $parentInfoDate['minutes'])->format($format);
	if ($baseTimeZone)
	{
		date_default_timezone_set($baseTimeZone);
	}
	CTimeZone::Enable();

	return $eventDate;
}